Case nitrification of steel



Patented Nov. 26, 1929.-

UNITED STATES PATENT OFFICE aueusrus 3. xINzEL, or BAYSIDE, mew YORK,Ass1enon. 'ro-nnmo METALLUR- GICL'L COMPANY, A CORPORATION or wear'vmemm case mrnrrrcarron or swam.

HoDrawing. Application filed. February 13, 1928,

, 5 ammonia.

This application is a continuation in art of my application, Serial No.214,447, led

alloys containing at least a critical amount August 20, 1927', whichrelates to methods of nitrogenizin articles consisting of ferrous ofvanadium to produce thereon a hard sur face containig vanadium andnitrogen.

Some of the ferrous materials which otherwise have desirable propertiesfor use in machine parts or other structural elementscannot be usedwhere they are subject to wear because a wearing surface of satisfactoryhardness cannot be given them economically. For such purpose, a surfacehardness of 700 Brinell is desired and a hardness of 900 Brinell orhigher is preferred.

I have discovered that surfaces of such degree of hardness can beproduced by nitrogenizing ferrous articles in which the vanadium contentis confined to the surface. I

have also discovered that the hardness of the nitrogenized coating canbe'regulated to a considerable extent by varying the content of vanadiumnot present as carbide in the surface layer. That is to say, when thecon tent of uniformly distributed non-carbide vanadium (designatedherein unbound vanadium) is insuflicient to bring about maximumhardening, the hardness of the nitrogenized coating can be increased byincreasing the concentration of unbound vana- 'dium in the surface layerof the article.

.The layer of hard'nitrogenized material is usually thin and thehardness is greater with larger amounts of unbound vanadium up to about0.5% of unbound vanadium. I have discovered that when the surfaces offerrous articles are enriched in vanadium'up to or beyond the criticalamount and then the articles are subjected to heat in the presence ofammonia or other nitrogenizing compounds at temperatures of about 500 to580 C. the desired surface hardness is produced without materiallyaffecting the properties of the core of the material.

Serial No. 254,162. Renewed June 24, 1929.

I usually prefer to remove carbon from the surface of the material, ifcarbon is present, and then to incorporate vanadium into the surfacebefore nitrogenizing, but decarburizing of the surface is not alwaysnecessary, and may be omitted where only a small addition of vanadium isrequired to produce the desired vanadium-containing hardeningconstituent. Decarburization may also be omitted where only a smallamount of carbon is present or where a relatively large addition ofvanadium is practicable to overcome the effect of carbon. I usuallyprefer to heat the article in the presence of hydrogen to effectthe-surface decarburizavanadium in a neutral or reducing atmosphere toaffect the introduction of vanadium,

but other means of decarburization and other means of incorporatingvanadium into the surface skin may be used. Decarburization alone willsufliciently increase the concentration of unbound vanadium in somecases where the article is composed of alloy relatively rich invanadium. I

My invention will be more fully understood by referring to the followingexamples. An article containing 0.5% vanadium; and 1.0 carbon was heatedin an atmosphere of hydrogen to 700 C. for 2 hours. Ammonia gas wassubstituted for the hydrogen and a temperature of 510 C. was maintainedfor about 16 hours.- The surface of this piece hada hardnesscorresponding to about 900 Brinell. Another piece of the same steelwastreated in'like manner inthe presence of ammonia without decarburizingthe surface. Much less hardness was im arted to the second piece than tothe piece in which the ratio of vanadium to carbon was increased.

The critical amount of vanadium as stated in my prior application isabout 0.3% to 0.5% unbound vanadium In the practice of my presentinvention, only the surface layer is enriched with vanadium to theextent required for producing the desired hardening effect when thearticle is nitrogenized.

The presence of vanadium in the original material is not necessary toproduce the desired hardening surface as will appear from thetemperature was reduced to 510 C. and

the heating was continued for 16 hdurs in the presence of ammonia. Thesurface of this piece was harder than 900 Brinell and there was nomaterial change in the properties of the core of the material.

Should I prefer to use aluminum instead of vanadium as the nitrifiableelement I may do so. A ferrous alloy may be aluminized in any desiredmanner but I prefer to heat the ferrous alloy in contact with aluminumpowder under neutral or reducing conditions at a temperature of about500 to 800 C. I have obtained satisfactory results when the ferrousalloy was treated with aluminum for one half to two hours at 550 C., butother temperatures and times may be used. If, by chance, theconcentration of aluminum in the surface becomes too great, I may reduceit to the desired value by soaking the ferrous alloy at a suitabletemperature, as for example about 900 C. The surface-aluminized ferrousalloy is then heated to about 500 to 580 C. in the presence of asubstance, such as ammonia, that yields nitrogen to the material. Thistreatment provides a hard nitrogenized compound only as a surface layerand the properties of the core of the material will not be detrimentallyaffected. The nitrogenization preferably is carried out at a temperaturenot materially above 580 (3., and under these conditions there is noserious embrittling of the interior portions of the member treated.

I have previously described the use of aluminum as a nitrifiable elementin my copending application Serial No. 209,184, filed July 28, 1927,from which this description is taken.

In the foregoing examples, the content of unbound vanadium in thesurface of the ar-' ticle was increased to the point where a very highdegree of hardness was produced when the piece was nitrogenized, butother degrees of hardness may be produced by regulating the times andtemperatures of treatment with the decarburizing agent, the vanadiumintroducing agent, and with the nitrogenizing agent.

a partial decarburization at the surface will produce an enrichment inunbound vanadium and a treatment with the nitrogenizing agent will thenproduce the desired hardening. Other modifications within the scope ofmy invention will suggest themselves to those skilled in the art.Therefore, I desire to be limited only by the prior art and by theinvent-ion as defined in the annexed claims.

I claim as my invention:

1. The method ofmaking a hardened surface coating on an article composedof fer rous material which comprises enriching the surface of thearticle in unbound vanadium and then subjecting the article to heat inthe presence of a material that yields nitrogen to the coating.

2. The method of making a hard surface coating on an article composed offerrous material which comprises heating in anonoxidizing atmosphere inthe presence of vanadium, thereby introducing vanadium in a surfacelayer and then heating in the presence of a material that yieldsnitrogen to the material.

3. The method of making a hard surface coating on .an article composedof ferrous material which comprises heating in an nonoxidizingatmosphere in the presence of vanadium, thereby introducing vanadium inthe surface lager and then heating to about 500 to 580 in the presenceof ammonia.

4. The method of making a hard surface coating on an article composed offerrous material which comprises decarburizing a surface layer,enriching said layer in unbound vanadium and then heating in thepresence of a material that yields nitrogen to the article.

5. The method of making a hard surface coating on an article composed ofa ferrous material which comprises successively heating the article inthe presence of a decarburizing agent and vanadium in a nonoxidizingatmosphere to temperatures sufficient to decarburize a surface layer andto introduce vanadium therein and then heating to about 500 to 580 C. inthe presence of ammonia.

6. The method of making a hard surface coating on an article composed of.a ferrous material which comprises heating the article in the presenceof hydrogen to a temperature sufiicient to remove carbon from at least asurface layer thereof, heating in the presence of vanadium to atemperature suflicient to introduce vanadium in the surface and thenheating to 500 to 580 C. in the presence of ammonia.

7. The method of making a hard surface coating on an article composed ofa ferrous material which comprises heating the article to about 700 C.for about 2 hours in the presence of hydrogen, heating in the presenceof vanadium to about 900 C. for 2 hours and then heating in the presenceof ammonia to about 500 to 580 C. for a time suflicient to make a hardsurface layer on the article.

8..The method of making a hard surface coating on anarticle containiniron, va-

nadium and carbon which comprises depriving only a surface layer of thealloy article of carbon ,to enrich the surface in unbound vanadium andthen heating in the presence of a material that ields nitrogen.

9. The method of with vanadium to an extent equal to or greater than thecritical amount suflicient to form a hardened nitrogenized compoundwhich comprises heating in the presence of a nitrogen containingcompound to about 500 to 580 C.

10. An articlecomposed -of a core of ferrous metal and a surface layerenriched with vanadium, said surface layer being integral with the coreand containing a constltuent that is formed by nitrogenizing aferrosvanadium alloy containing vanadium in solid solution.

11. An article composed of a core of ferrous metal and a surfacelayerenriched with vanadium, said surface layer being integral with the coreand containing a constituent that is formed by nitrogenizing aferro-vanadium alloy containing vanadium in excess of that required toform carbides.v

12. An article composed of a core of ferrous metal and a surface layerenriched with vanadium and .containing unbound vanadium in excess of.3%, said layer'being integral with the core and containing nitro; gen,and said core being softer than the exterior portion.

13. The method of making a hardened surface coating on an articlecomposedof a ferrous material which comprises modifying the surface ofthe article with respect to its content of a nitrifiable alloyconstituent so as to produce a surface layer containing at least acritical amount of the nitrifiable alloy and subjecting the article toheat in the presence of a material that yields nitrogen to the coatin14. An art1cle composed of a core of ferrous material and a surfacelayer enriched with a nitrifiable alloy constituent, said surface layerbeing integral with the core and containing a constituent that is formedby nitrogenizing a ferrous alloy containin a nitrogenizable constituentin solid solutlon.

I testimony whereof, I afiix in Si ature.

AUGUSTUS B. I ZEL.

making a hard surface coating on an article composed of ferrous 7material which has a surface layer enriched

